Liquid-crystal display devices have been used in various fields as typical display devices for personal computers and word processors, as display devices for television sets and car navigation systems, and further for projection-type display devices, because of their thinness, lightness, and low power consumption. Specifically, active-matrix liquid-crystal display devices have display pixels electrically connected to switching elements, and research and development thereof have been made actively because they provide good display images without crosstalk between adjacent pixels.
In recent years, display devices having the effective display area with an aspect ratio of 4/3 have been being replaced with display devices having the effective display area elongated in the direction of horizontal scanning line, such as the effective display area with an aspect ratio of 16/9. This aspect ratio provides the user with a visually wide screen.
There have been several methods of causing a liquid-crystal display device having the effective display area with an aspect ratio of 16/9 to display the video signal having image information with an aspect ratio of 4/3.
For example, one known method is to sample sequentially the video signal Video with an aspect ratio of 4/3 shown in FIG. 12A sequentially all over the effective display area with an aspect ratio of 16/9 on the basis of a specific sampling clock and display the result as shown in FIG. 12B. With this method, a special circuit configuration is not needed, but the aspect ratio of the video signal with an aspect ratio of 4/3 is not be reproduced faithfully, which results in the image elongated in the direction of horizontal scanning line as shown in FIG. 12B.
Hereinafter, a method of displaying the video signal all over the effective display area is referred to as the full display mode.
Another known method is to divide the effective display area with an aspect ratio of 16/9 into a display area with an aspect ratio of 4/9 and a display area with an aspect ratio of 12/9 (=4/3) as shown in FIG. 12C or 12D and display the video signal Video having image information with an aspect ratio of 4/3 in the display area with an aspect ration of 12/9 (=4/3). Hereinafter, this display method is referred to as the left-aligned or right-aligned mode, depending on the display position.
With this method, however, since the sampling period corresponding to the display area with an aspect ratio of 12/9 (=4/3) requires at least 0.8 H for one horizontal scanning period (1 H) in the NTSC system, the sampling period corresponding to the display area with an aspect ratio of 4/9 is 0.2 H or less. Therefore, the video signal has been sampled sequentially and displayed on the basis of the video signal previously image-processed all over the effective display area by image processing techniques using a frame memory. This has prevented the device from being produced at low cost.
Under the circumstances, in Japanese Patent Application No. 9-227656, the applicant has proposed a device and method for realizing the above display easily by making the frequency of the sampling clock different in each horizontal scanning period. This display method requires no frame memory.
Furthermore, as shown in FIG. 13A, it is needed to divide the effective display area having an aspect ratio of 16/9 into a display area having an aspect ratio of 2/9 and a display area having an aspect ratio of 12/9 (=4/3), and display the video signal Video whose image information has an aspect ratio of 4/3 in the display area having an aspect ratio of 12/9 (=4/3) in such a manner the image is located in the center.
As shown in FIGS. 13B to 13D, the following display modes are needed: the left-aligned/full composite display mode, a combination of the left-aligned display mode and full display mode; the right-aligned/full composite display mode, a combination of the right-aligned display mode and full display mode; and the centering/full composite display mode, a combination of the centering display mode and full display mode.
Furthermore, as shown in FIG. 13E, the following display modes are needed: the irregular composite display mode obtained by combining the centering display mode with the full display mode irregularly (hereinafter, referred to as the centering/full irregular composite display mode); the irregular composite display mode (not shown) obtained by combining the left-aligned display mode with the full display mode (hereinafter, referred to as the left-aligned/full irregular composite display mode); and the irregular composite display mode obtained by combining the right-aligned display mode with the full display mode (hereinafter, referred to as the right-aligned/full irregular composite display mode).
In the composite display mode and irregular composite display mode, because the drive timing differs from area to area, for example, in the horizontal select period or polarity inverting timing, it has been difficult to display images without carrying out a special image process.